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The last major architectural feature of a parallel machine is the network or design of hardware/software connecting processors and memories together.
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Bus: All processors (and memory) connected to a common bus or busses.
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Memory access fairly uniform, but not very scalable due to contention
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Bus machines can be NUMA if memory consists of directly accessed local memory as well as memory banks accessed by Bus. The Bus accessed memories can be local memories on other processors
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Switching Network: Processors (and memory) connected to routing switches like in telephone system.
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Switches might have queues and "combining logic", which improve functionality but increase latency.
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Switch settings may be determined by message headers or preset by controller.
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Connections can be packet-switched (messages no longer than some fixed size) or circuit-switched (connection remains as long as needed)
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Usually NUMA, blocking, often scalable and upgradable
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